runtime: sleep a bit when waiting for running debug call goroutine

Without this CL, one of the TestDebugCall tests would fail 1% to 2% of
the time on the android-amd64-emu gomote. With this CL, I ran the
tests for 1000 iterations with no failures.

Fixes #32985

Change-Id: I541268a2a0c10d0cd7604f0b2dbd15c1d18e5730
Reviewed-on: https://go-review.googlesource.com/c/go/+/205248


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>